1. Field of Invention
The present invention relates to vibrating motors using piezoelectric actuators. More particularly the present invention is directed to vibrating motors and methods of mechanically mounting the piezoelectric actuators in motors to transfer vibrational energy through an attached plate and flexible membrane to a work surface.
2. Description of the Prior Art
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field.
A typical prior ceramic device such as a direct mode actuator makes direct use of a change in the dimensions of the material, when activated, without amplification of the actual displacement. Direct mode actuators typically include a piezoelectric or electrostrictive ceramic plate sandwiched between a pair of electrodes formed on its major surfaces. The device is generally formed of a sufficiently large piezoelectric and/or electrostrictive coefficient to produce the desired strain in the ceramic plate. However, direct mode actuators suffer from the disadvantage of only being able to achieve a very small displacement (strain), which is, at best, only a few tenths of a percent.
Indirect mode actuators are known in the prior art to provide greater displacement than is achievable with direct mode actuators. Indirect mode actuators achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer. Prior flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, they can exhibit several orders of magnitude greater displacement than can be produced by direct mode actuators.
The magnitude of the strain of indirect mode actuators can be increased by constructing them either as xe2x80x9cunimorphxe2x80x9d or xe2x80x9cbimorphxe2x80x9d flextensional actuators. A typical unimorph is a concave structure composed of a single piezoelectric element externally bonded to a flexible metal foil, and which results in axial buckling or deflection when electrically energized. Common unimorphs can exhibit a strain of as high as 10% but can only sustain loads that are less than one pound. A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements. Electrodes are bonded to each of the major surfaces of the ceramic elements and the metal foil is bonded to the inner two electrodes. Bimorphs exhibit more displacement than comparable unimorphs because under the applied voltage, one ceramic element will contract while the other expands. Bimorphs can exhibit strains up to 20% (i.e. about twice that of unimorphs), but, like unimorphs, typically can only sustain loads which are less than one pound.
A unimorph actuator called xe2x80x9cTHUNDERxe2x80x9d, which has improved displacement and load capabilities, has recently been developed and is disclosed in U.S. Pat. No. 5,632,841. THUNDER (which is an acronym for THin layer composite UNimorph ferroelectric Driver and sEnsoR), is a unimorph actuator in which one or more pre-stress layers are bonded to a thin piezoelectric ceramic wafer at high temperature. Cooling the composite structure asymmetrically stress biases the ceramic wafer due to the difference in thermal contraction rates of the ceramic layer and the pre-stress layers (or substrate). In other words, as the substrate(s) and adhesive cool they contract more than the ceramic to which they are bonded. This places the ceramic layer under compression and the substrate in tension. Because the ceramic layer in compression is bonded to the substrate(s) in tension, the assembled actuator assumes its normal arcuate shape. This prestress condition which compresses the ceramic layer enables the ceramic to be less susceptible to cracking as well as increasing the amount of deformation and resultant strain that the actuator may experience.
In operation a THUNDER actuator may be energized by an electric power supply via a pair of electrical wires which are typically soldered to the metal prestress layers (substrates) or to the electroplated faces of the ceramic layer. When a voltage of a first polarity is applied across the ceramic layer, the ceramic contracts (in the direction of the tension in the substrate), which causes the actuator to relax and flatten (position 99 in FIG. 4). When a voltage of an opposite polarity is applied across the ceramic layer, the ceramic layer expands (increasing the tension in the substrate), which causes the actuator to become more concave (position 101 in FIG. 4). By applying an alternating voltage, the ceramic layer in the actuator can cyclically contract and expand, which causes the actuator to alternately become more and less concave (as illustrated by positions 99 and 101 in FIG. 4).
In practice, these actuators 100 have been used to directly drive a pressure plate 8 or other mechanism in a prior art cyclic motor as shown in FIG. 1. Typically, the convex face 100a of the actuator 100 would directly push (in the direction of arrow 7) against a plate 8 at the lowest point of its curvature, and the plate 8 would maintain contact with the actuator 100, returning to its rest position through the use of a spring mechanism 6.
FIG. 2 illustrates another device using multiple stacked actuators 100. Each actuator 100 has its edges 11 mounted in slots 67 in the sidewalls 70 of a housing 72. The actuators 100 and their electrical connections are electrically isolated from each other using spacers 33, typically TEFLON(trademark) insulators, that are mounted to a spring 6 biased drive shaft 32. The drive shaft 32 may be further mounted to a pressure plate 8 or other motion translating means (not shown).
A problem with the above described mounting methods for a direct drive actuator is that the force against the actuator 100 was concentrated on one point, or at least in a very small area of the actuator 100. This would cause the ceramic 10 in the actuator 100 or the whole actuator 100 to break due to point load concentration. The actuator 100 would then lose most of its effectiveness because it could not generate as much force or displacement with a cracked ceramic 10.
Another problem with prior art actuator mounting methods is that a single actuator typically could not generate sufficient force for higher output applications. This is especially true of applications where the pressure plate against which the actuator acted was spring mounted. The actuator dissipated a large amount of its useful force in trying to overcome the spring mechanism. The force generated for some applications would also fracture the ceramic layer of the actuator.
Another problem with prior art actuator mounting methods is that even a stack of actuators acting against a pressure plate typically could not generate sufficient force for higher output applications.
Another problem in applications using multiple, stacked actuators was that the spacers add weight to the motor as well as opposing the motion of the actuators, dissipating the useful force and displacement in the stacked actuators. The actuators also dissipated a large amount of their useful force in trying to overcome the spring mechanism. The force generated for some applications would also fracture the ceramic layer of the actuator.
Another problem with prior mounting methods for single or multiple actuators was that where a lightweight motor was desired, the use of a housing in which to mount actuator edges would add extra weight to the motor.
The present invention provides a vibrational motor and a method of mounting piezoelectric actuators or stacks of piezoelectric actuators on the vibrational motor. Specifically, the present invention provides a vibrational motor in which, relative to prior art devices, the weight and dissipative forces are minimized, load distribution is more uniform and mechanical mounting is more secure. In the preferred embodiment, THUNDER type piezoelectric actuators are attached at their edges to top and bottom mounting members, and this motor assembly is attached to the blade of a vibrational tool. This vibrating tool may advantageously be used to modify the texture or character (i.e. the xe2x80x9cfinishxe2x80x9d) of a surface of a work material or for other purposes. The vibratory action of the tool is generated by one or more piezoelectric actuators which, when energized, vibrate at the frequency of the applied voltage. The piezoelectric actuators are mounted in a motor assembly that is attached to a reaction mass, a blade/plate and a flexible membrane. In the preferred embodiment of the invention, the vibrations are transferred from the motor and attached mass through the blade and flexible membrane at the bottom of the tool and into plastic concrete work material. This vibration causes air and water to rise to the surface of the concrete creating a slurry, which is desirable for producing a smooth surface finish. The motor assembly is sufficiently light that a reaction mass may be attached to it to tune the amplitude and resultant force of the vibrations which are transferred to the blade of the vibrational tool. The reaction mass also ensures that the majority of vibrations are transferred in the appropriate direction, i.e., downward to the work surface. This, coupled with the lightweight design and other characteristics described hereinbelow, makes the tool very easy to handle and operate.
Accordingly, it is a primary object of the present invention to provide a mounting for a piezoelectric actuator that allows for the production of the force and displacement for motor applications.
It is a further object of the present invention to provide a device of the character described in a lightweight piezoelectrically actuated vibrating tool.
It is a further object of the present invention to provide a device of the character described in which multiple actuators are used to produce high force in a drive mechanism.
It is a further object of the present invention to provide a device of the character described in which loads driven by the actuator are effectively distributed across the structure of the actuator to avoid fracture of the ceramic element of the piezoelectric actuator.
It is a further object of the present invention to provide a device of the character described in which stacked actuators are mounted without adding undue weight.
It is a further object of the present invention to provide a device of the character described in which losses in force and displacement are minimized through effective coupling to a drive mechanism.
It is another object of the present invention to provide a device of the character described in which the mechanical mounting of the actuators is particularly secure.
It is another object of the present invention to provide a device of the character described which is at the same time compact, light in weight, and of an extremely simple and uncluttered design.
It is another object of the present invention to provide handheld concrete/cement working tools of an automatically vibrating variety wherein a substantial vibratory energy is imparted to the concrete surface.
It is another object of the present invention to provide a device of the character described in which there is minimal vibration transmitted through the handle (and subsequently to the operator) in proportion to the amount of vibration transmitted through the bottom of the device and into the concrete.
It is another object of the present invention to provide a device of the character described wherein a reaction mass is provided to tune the amplitude to the vibratory energy imparted through the tool.
It is another object of the present invention to provide a device of the character described wherein
It is another object to provide a modification of the present invention in which the vibratory energy is imparted in the frequency range of 50 to 500 hertz.
It is another object to provide a modification of the present invention in which the frequency of vibration is easily user-modified.
It is another object of the present invention to provide a device of the character described which is battery powered.
It is another object of the present invention to provide a device of the character described wherein multiple motors may vibrate the vibrational tool, or multiple tools may be attached to form a larger one.
It is another object of the present invention to provide a device of the character described wherein the blades of individual tools operate synchronously while being linked with a flexible membrane.
Further objects and advantages of this invention will become apparent from a consideration of the drawings and ensuing description thereof.